G. E. BOYD,E. W. GRAHAM AXD Q. V. LARSON
300
AT,,,*=
solvated monomer
AF
=
-6.1 kcal./mole
-6.1 kcal./mole
AHm0= f1.9
A& = 0 A& = +20.3 cal./mole deg.
Vol. 66
~ ~ & $ ~ d
kcal./mole
solvated monomer (X,= c.m.c.)
/’
.;i”AE, Af,
AS,
This diagram clearly shows the relationships between the thermodynamic quantities frequently cited for micelle formation. Note that the heat and entropy terms for micelle formation are positive and remain positive over the temperature range 1-50’, One must conclude, since the standard free energy is negative and the heat term is positive, that it is the entropy change that is responsible for the formation of amine oxide micelles. A positive entropy change indicates increased randomness. It does not appear likely that increased disorder would result from the aggregation of surfactant monomers into micelles, therefore one looks to solvation changes to explain the positive ASh. Goddard and B e n ~ o nstudied ~ ~ the variati6n of sodium alkyl sulfate c.m.c.’s with tEmperature over the range 10-55’ and found that AH, changes sign beiween 25-30’. An explanation of the positive AH, and AS, values was given by Goddard, Hoeve (34) E. D. Goddard and G. C. Benson, Can. J . Chem., 36, 986 (1957).
= 0 = +1.9 kcal./mole = +6.3 cal./mole deg.
and BensonZ6based on modification of the postulated water structure surrounding the monomer hydrocarbon chains. The ordering of water (“iceberg” formation) by relatively large organic ions has been discussed previously by Frank and coworkers. 351 Although solvent modification does seem likely to occur when micelles are formed, an exact description of the solvent’s role in micelle formation still is lacking. Information on the degree of association of water with monomers and micelles and on the heat of monomer solvation would be helpful in understanding solvent participation in micellization. Acknowledgments.-The author is indebted to Mr. P. Baumgardner who assisted in making the light scattering measurements. S. Frank and W. W. Evans, J . Chem. Phys., 13, 507 (1945). (36) H.8. Frank and W. Y. Wen, Discussions Faraday SOC.,24, 133 (1958). (35) H.
RECOIL REACTIONS WITH HIGH INTENSITY SLOW NEUTRON SOURCES. IV. THE RADIOLYSIS OF CRYSTALLINE ALKALI METAL BROMATES WITH y-RAYS BY G. E. BOYD,E. W. GRAHAM AND Q. V. LARSON Oak Ridge hratioml Laboratory, Oak Ridge, Tennessee Receized Beplember 16, 1961
The radiolysis of the alkali metal bromates by Coo0 ?-rays was effected to decompositions greater than three mole per cent. The amounts of bromate ion decomposed increased linearly with dose a t first, but subsequently the dependence became non-linear for all salts except LiBr03. The initial IO0 e,v. radiolytic yields or “GO’,values for bromate decomposition were: 0.31, 1.46, 1.71, 2.33, 3.4 and 5.1 for LiBrOa, NaBrOe, f(BrOa, RbBrOa, CsBr03 and TlBrOa, respectively. The yields increased only slightly with temperature between - 195 and 85”,and for CsBrOg were almost independent of the dose rate from 8.8 x 1014to 7.6 x 10’6 e.v. g.-1 set.-'. The radioIy$is of bromate ion gave bromite, hypobromite, bromide and oxygen gas in amounts which varied with the alkali metal cation in the salt and with the total dose absorbed. Virtually all theoxidiaing fragments produced in the crystalline salts by irradiation could be removed by thermal annealing. The radiolytic yields for bromate decompositio could not be correlated either with the thermodynamic stability of the salts or with their isothermal decomposition rates ogserved in the absence of radiation. However, an exponential dependence of “GO” on the crystal “free space” was found, and a mechanism for the decomposition of the molecular bromate ion could be proposed.
This study deals with the radiation chemistry of molecular ions in crystals, and it is an extension of our earlier researcheslatb82on the decomposition of the bromate ion in crystalline potassium bromate when this compound was exposed to various types of energetic nuclear radiation. The chief concern has been in determining the importance of crystal structure and binding to the radiolysis of the aIkali (1) (a) G.E.Boyd, J. W. Cobble and 8. Wexler, J . Am. Chem. Soc., 74, 237 (1952): (b) J . W. Cobble and G. E. Boyd, ihid., 74, 1282 (1952). (2) G.E. Boyd and J. W. Cobble, J. Phys. Chem., 63, 9lQ (1959).
metal bromates by y-rays, and in determining the details about the radiolytic decomposition of the bromate group. It was of interest, also, to discover if any of the other bromates were more radiation stable than the potassium salt, if susceptibility to radiolysis were dependent on the manner in which the crystals mere prepared or On the impurities they contained, and if conditione during the irradiations such as temperature could be chosen to minimize decomposition. The decompositions produced by Coao y-rays were measured because it was expected that the
Feb., 1962
RADIOLYSIS OF CRYSTALLINE ALKALIMETALBROMATES WITH Y-RAYS
30 1
evolution of heat, and above their melting points the reaction became strongly exothermic. This behavior is consistent with the standard heats, AHodes, for decomposition to alkali bromide and oxygen gas. The standard free energies of decomposition, AF0deo, (Table I, col. 5) were derived from estimations of the free energiee of formation, AFDf,based on calorimetric measurements of the heats of solution of the crystalline bromates.6 The ApDdeo. values reveal that all of the salts are thermodynamically unstable at 298.1 OK. Despite this instability the crystals did not decompose a t an appreciable rate until they were heated well above their melting points. This behavior suggests that the thermal decomposition of bromate must occur with an appreciable activation energy. Columns 6 and 7, Table I , summarize published information to be employed later in this paper on the crystal symmetry and X-ray density of the alkali metal bromates. Anhydrous LiBr03, whose structure previously was unknown, was demonstrated as orthorhombic in a special study. X-Ray diffraction powder patterns were taken on all of the preparations, and an excellent agreement with the N.B.S. Circular 539 crystal constants was obtained. The structures of KBrOa, RbBrOa, CsBr03 and TlBrOa were hexagonal-(trigonal) and hence these compounds are isomorphous. Irradiation of Samples .-The irradiations were conducted in the 10.5”. X 10.5” X 12” cavity of the ORNL Cobalt Storage Facility where dose rates of 3.4 to 5.1 X 10’6 e.v. g.-’ sec.-l in water were observed during the period of our investigations. The temperature of the chamber was uniform and varied between 80 and 105’. The thermal decom osition of the crystalline bromates was negligibly smalpat 105”. Approximately 5-g. amounts of salt contained in glass-stoppered, 1.16-cm. diameter by 4.0 cm. high glass vials were irradiated a t the center of the floor of the chamber in a fixed geometry arrangement. Experimental Dose rate measurements were made for almost every irPreparation of Anhydrous Compounds.-The lithium, radiation. Irradiations a t - 19.5’ were performed in 500sodium and potassium bromates employed were either spec- ml. stoppered wide-mouth Dewars filled with liquid nitrotrochemically pure commercial products or reagent grade gen. The samples were impended in the center of the bath. chemicals. Rubidium and cesium bromate were syntheThree other Co60 irradiation facilities were employed in sized starting with the pure carbonates or with CsCl. Pro- studies of the dose rate dependence of the radiolysis: nomcedures described elsewhere6 were followed. Thallium inal 300,O 1100 and lO,OOO7 curie sources giving dose rates bromate was prepared by a double decomposition reaction in water of 1.22 X. 1016, 7.45 X 10’6 and 1.08 X 1017 e.v. between T1NO3 and KBrOa. The TlN03 was made by dis- g.-1 sec.-1, respectively. solving thallium metal (American Platinum Works) in conDosimetry.-Gamma-ray dose rate measurements were centrated nitric acid and recrystallizing once from water made with approximately 0.025 M ceric sulfate solutions containing IINOa to minimize hydrolysis. The slightly 0.4 M in HL304.8 This solution was prepared from caresoluble TlBrOs formed was separated by filtration and washed fully purified reagents and water and was “aged” by heating twice with cold water. overnight at 90”. It was stored away from light and was The bromide contents of the alkali metal salts were re- stable over many months.9 I n the measurements 15 ml. duced to acceptably low levels (Table I ) by recrystalliza- wa.s placed in glass tubes (2.5 cm. diameter by 4.5 cm. deep) tions from water. All compounds were dried in air a t 110’ which were closed-off with Bakelite screw-top caps. Dose and stored away from light in closed vessels. The formation rates were computed from the equivalents of Ce(1V) reof LiBrOs.H*O was prevented by storing the anhydrous duced, as determined by potentiometric titration with standsalt over a desiccant; further, the loading of samples into ardized FeS04solutions,and the yield values (eq. per 100e.v.) paraffin-sealed glass-stoppered vials for irradiation was con- given by the equation:’O G(CeII1) = 2.35 0.37(Ce(IV))l/z, ducted in a dry box, The crystals appeared to be stable which holds for concentrations from to lo-’ M . The over many months, excepting TIBrOa which decomposed reduction of the initial Ce(1V) concentration by radiation slowly on standing in air. Both RbBrO, and the CsBrOa-1 varied from 20 to 60%. preparations were found to be quite free from the other precision of the dose rate measurements (including alkali metals by flame spectrophotometric analysis. The theThe reproducibility of the exposure geometry) appeared to CsBrOz-2 preparation, however, was contaminated with lie between one and three per cent. as indicated by results 4.6% potassium and 0.73% rubidium by weight. obtained a t frequent intervals. Characterization of the Alkali Metal Bromate PreparaThe precision of the analytical determinations of the tions.-Results from several measurements to characterize amounts of Ce(1V) reduced was about 0.2%. The relithe alkali metal bromate preparations described above are ability of the Ce(1V) dosimetry measurements was estabsummarized in Table I. The melting points listed in column lished by periodic comparisons with the dose rate obtained 3 were determined by differential thermal analysis (DTA) with the ferrous sulfate (Fricke) dosimeter. techniques using a heating rate of 9.5 degrees per minute. Analysis for Radiolytic Products.-Weighed amounts Good agreement with literature values for RbBrOa and usually (ca. 1.0 g.) of irradiated crystals were analyzed for CsBrOa was obtained, but not with NaBrOa (381’) and bromide ion after dissolving them in distilled water conKBr03 (434’). No melting point temperature for LiBrO, taining excess 0.1 N sodium arsenite and allowing the soluhas been reported hitherto. The DTA studies showed that tion to stand for at least 30 min. After making up to 25.00no phase transitions occurred in any of the alkali metal bromates below their melting points. Above 200” the de(6) J. A. Ghormley and C. J. Hochanadel, Rev. Sci. I n 5 t T , aa, 473 composition of the compounds was accompanied by the (1951).
dependence of these on the absorbed dose, etc., would afford a basis for comparison with the more complex radiolytic effects produced by neutron reactor radiations to be reported in a subsequent paper. I n addition, Coeoy-radiations are preferred for fundamental studies, because the methods for radiation dose measurement on them are well established in contrast to the situation with respect to the dosimetry of pile radiations where appreciable uncertainty persists. There are no publications which report other than the formation of bromide in the radiolysis of the alkali metal bromates, or which attempt to give a mechanism for their radiation decomposition. The production of chloride, hypochlorite and chlorite has been observed in preliminary investigations of the decomposition of KC103 by X-rays,a and recently Co60 y-rays have been reported to give chlorite, chloride and oxygen gasa4 A radiolytic chlorite yield of 1.2 molecules per 100 e.v. was observed, which could be reduced to 0.8 on heating the irradiated salt a t 200”. The conversion of chlorite to chloride and oxygen was reported to be responsible for the yield reduction. The radiolysis of the molecular chlorate ion was interpreted as proceeding through intermediate C103free radicals which decomposed either to chlorite or to chloride.
+
(3) H. G. Heal, Can. J . Chem., 31, 91 (1953). (4) A. S. Bakerkin, “The Action of Ionizing Radiation on Inorganic and Organic Systems,” Moscow, Acad. Sei. U.S.S.R.Press, 1958, p. 187. (5) G. E. Boyd and E‘. Vaslow, J . Chem. Eng.Data, in press.
(7) W. Davis, Jr., “The Chemical Technology Division (2060 Source,” ORNL-CF-60-3-82, March, 1960. (8) S. I. Taimuty, L. H. Towle and D. L. Peterson, Nucleonics, 17, 103 (1959). (9) J. T. Harlen and E. J. Hart, ibid., 17, 102 (1959). (10) C. J. Hochanadel, private communication, June, 1959.
302
Vol. 66 TABLE I CHARACTERIZATION OF ALKALIMETALBROMATE PREPARATIONS Salt
Residual bromide content (p.p.m.)
M.p., o c .
AHodec. AFQdec. kcal. mole-1
------Crystal Symmetry
structure---X-Ray density
LiBrOa-1; -2 19; KBr03 > CsBr03. An aliquot of KBr03 heated for 24 hr. at 325' was titrated for oxidizing power and none was found; the thermal decomposition appeared to go entirely to bromide and oxygen gas. Role of Surface Area in Thermal and Radiolytic Decomposition.-A possible dependence of radiolysis on surface was investigated using the RbBrO3-1 preparation which was obtained in large crystals. An aliquot was pulverized and irradiated together with some of the original preparation to a dose of 0.478 X 10Qe.v. mole-': 668 =k 4 and 671 f 1 p.p.m. Br- ion were produced, respectively, indicating that the extent of surface was unimportant. I n another experiment KBr03-2 was irradiated to a dose of 5.51 X 1 0 2 3 e.v. mole-1 to give a decomposition of 1.15 mole %. A measurement of its specific surface by krypton as adsorption at -195" gave 0.022 m.2 g.-1; the same%Br03 before irradiation showed an area of 0.0226 f 0.0005 m.2 g.-1. A surface area determination on an aliquot of KBrOa-2 heated at 322" for 16 hr. to give a 1.19 mole 70decomposition gave 0.0364 I 0.0020 m.z g.-I suggesting, in contrast to radiolysis, that pyrolysis occurred on external surfaces. Determination of Oxygen Gas in Irradiated Salts.-The irradiated salts were found to evolve gas in perceptible quantities when they were dissolved in water. Accordingly, several quantitative measurements (Table 11) were made of the amounts of oxygen released using a gas chromatographic method. Extensively irradiated crystals were placed in a closed vessel connected with a vacuum line and were dissolved in de-gassed water after the system had been evacuated. The gases liberated were transferred under low pressure ( KBr03 > CsBr03, whereas the reverse order held for radiation decomposition. (c) Thermolysis produced an increase in the surface, but no detectable change in area occurred in the radiolyzed crystals. Further, radiolytic yield did not appear to depend on particle size. (d) The alkali metal bromates are colored by exposure to y-rays, and oxidizing fragments are produced in them. KO coloration nor oxidizing fragments were observed in the thermally decomposed compounds. The foregoing observations have led to the view that thermal decomposition takes place largely a t the crystal surface, possibly through the formation of nuclei of alkali metal bromide follo~edby an interface reaction between this product and bromate. Radiolysis, in contrast, must occur mainly a t random in the crystal lattice or possibly in the vicinity of defects. Support for the hypothesis that the mode of radiolytic decomposition of the alkali metal bromates must be dependent on crystal lattice properties has been found in the apparent correlation of their initial yields (Go values) with the “crystal free space.” The latter quantityI3 may be defined as the difference between the volume per mole of crystal derived from the X-ray density and the volume per mole of constituent ions. The “free space” volumes per molecule of alkali metal bromate plotted in Fig. 3 were estimated using the densities in Table I. The volume per bromate ion was computed as 28.7 A.3 from the Br-0 bond distance19 and the radius*Oof Br+j. The volumes of the alkali metal cations were derived from the accepted crystal radii. The “free space” in the alkali metal nitrate crystals was estima$ed in a similar manner using a volume of 19.6 A.3 for the nitrate ion. Initial radiolytic yield values a t 25’ for the decomposition of the nitrates by Co60 y-rays were taken from independent sources.21322The exponential dependence (Fig. 3) of Go on the “free space” over a ten-fold range for the alkali metal bromates and over nearly 100-fold for the nitrate is of interest because it emphasizes the importance of crystal environment to radiolytic processes. Furthermore, the existence of such an empirical relationship might be expected from an elementary model for the dependence. However, the correlation noted in Fig. 3 may be subject to limitations: For example, the Go value for TlBr03, which possesses nearly the same “free space’’ as RbBrOs, (19) “Interatomic Distances,” The Chemical Sorioty, London, 1958. (20) L. H. Ahrens, Geochim. Cosmoehzm. Acta, 2, 155 (1952). (21) C. J. Hochanadel and T. W. Davis, J . Chem. Phys., 27, 333 (1957). ( 2 2 ) J. Cunmngharn, J . Phvs. Chem , 66, 028 (1961).
Vol. 66
was much higher than expected from Fig. 3. Additionally, the Go for AgN03 has been found1’ to fall below the smooth curve relating the alkali metal nitrates, whereas the curve for the alkaline earth nitrates lies above the latter. On the other hand, the high GOvalue observed with T1Br03 may have been caused in part by the thermal decomposition of this highly unstable compound, and it has been suggested” that the low value for AgX03 results from the efficient trapping and degradation of energy by silver ions. The apparent exponential dependence of Go on crystal free space is reminescent of the wellknown exponential dependence of reaction rates in solution on pressure.23 The analogy between these processes and the case in hand is re-enforced by the observation that the disruption of a bromate ion to yield bromide plus three oxygen atoms gives a 37.3 A. volume increase. Alternatively, decomposition to give oxygen molecules would be accompanied by a volume increase of 58.6 A.3 per ion. The amount of activation energy required for the diffusion of radiolytic oxygen from the site of decomposition should be proportional to the strain it causes in the crystal; this strain, for a given volume increase, \Till be the greater the smaller the interstitial space in the lattice and the greater the crystal binding. State of Dissociation and Fate of Absorbed Energy.-The first step in the radiolytic process, of course, is the absorption of energy which subsequently can be utilized in a chemical process. For small decompositions it seems reasonable to assume that the breek-up of excited or ionized bromate ions occurs a t widely separated lattice sites in the crystal; otherwise, it is difficult to understand the apparent importance of the crystal “free-space” noted above. Exciton migration to lattice defects or special trapping centers where a preferred decomposition of bromate might occur would afford an alternative mechanism. This alternative is regarded as an unlikely one, however, because an exceptionally large defect concentration would be required to sustain the initial radiolytic rate, which is estimated as 4.7 X 1015 bromate ions per ~ m per . ~sec. in CsBr03-l exposed to a dose rate of 3.2 X 1016 e.v. g.-I sec.-l. If, for example, the energy required to produce a lattice vacancy a t room temperature is taken as one e.v., the concentration of Schottky defects will be only 4 X lo6 per C M . ~ . An appreciable fraction of the electronically excited bromate ions formed in the lattice must decompose. Thus, for CsBr03-l it can be estimated that roughly 30 e.v. of energy is absorbed per Br03- ion. If the first excited electronic state of the ion lies at 4.6 e.v. above the ground state, as is indicated ,by the strong optical absorption band at 2700 A. found with aqueous solutions, then at least 15yo of the excited bromates decompose. Mechanism of Decomposition of the Molecular Bromate Ion.-Assuming that the y-ray energy (23) S.Glasstone, K. Laidler and H. Eyring, “The Theory of Rate Prooosses,” McGraw-Hill Book Co., Inc.. N e w York, N. Y . , 1941, pp. 470-474.
absorbed divides approximately equally into cxciting and ionizing bromate ions, the primary radiolytic :wt may be depicted as hv Br03-*
r+
1
~r0~--I
1 hv
BrOa
L +
+ e-
(1-w
(1B)
The dccompositiori reactions following cxcitation arc', then, formally r+
+ 0, or, Ur02 + 0 -
(2h)
+
(2B)
UrOL-
I J .
Br0-
BrOd-*-----+
1 . I+ Br-
L+-
0
0 2
+
0 2
(ZC)
At)struction rwctioiis BrO3-
+ 0 +BrOl- + O r ( g ) ,
(3)
rccombination reactions
+ 0 +02(g) Br02- + +Br03- + 0 0
0 2
(4A)
The over-all bromate decompositioii, howcver, appeared to be almost independent of these variablrs (Tables V and VI), so that it must be concluded that the formation of Br02- via (3) was relatively minor. (3) The occurrence of reaction 4A is supported by the fact that oxygen gas is released either on dissolving or heating the radiolyzed crystals, and by the observation of microscopic bubbles of gas formed in the body of the cryst@. Oxygen atoms are sufficiently small (ca. 12 A.3 per atom) that they should diffuse through the crystal easily. The concentration of uncombined atoms should be extremely small; reaction (4A) is highly exothermic. (4) Reactions 4B and 5 have been postulated to explain the fact that radiolytically produced Oxidizing fragments in the crystal can be removed by thermal annealing. Reaction 4B must havt occurred to a relatively minor extent as only a small fraction of thc Eragmants recombined to give bromate on heating. Reaction 5 may only represent the net of reaction 2A to give BrO2 0followed by
+
+
BrOz -+ Br BrfO-+Br-+O
(4B)
:nid t hcrmal decompositioii reactions
0 2
(5A) (5B)
Bromine dioxide is known to be quite unstable and to exist only below -40". (5) It will be assumrd that reaction 4B can must be considered in addition because of their possible role in determining the net yields of gas proceed by radiation excitation of the trapped 0 2 molecules (exciton transfer) held under high presand oxidizing fragments. The experimental evidence bearing on the occur- sures within the crystals. This reaction would account, a t least in part, for the non-linear dcrence of these reactions may be examined (1) The analytical determiiiations of Oxidizing pendence of bromate ion radiolysis on dose (Fig. 1) fragments, of bromite and hypobromite in radio- for large absorbed doses. Evidence for this kind lyzcd CsBrOa-1 and of bromide by argentiometric of a radiation annealing reaction has been puba KBr pellet was irradiated in titration and by X-ray diffraction suggest that lished reactions 2A-C, inclusive, may occur. Bromite an atmosphere of dry oxygen with 1.5 MeV. and hypobromite, which were observed only in electrons to a dose of about lo2' e x . g.-l and cxaqueous solutions, may have been formed by the amiricd for its infrared absorption. In addition reaction of trapped bromate (or other) free radicals to an absorption band a t 1440 cm.--' (which could with water on dissolving the radiolyzed crystals. have been from KBr0) the principal band for The formation of a relatively large number of free KBrOa a t '795 cm.--l was obscrvrd. The potential radicals would have been required, however, and importance of exciton trapping by radiolytic it seems more likely that Br02- and BrO- were products in determining the kinetics of the radiapresent in the crystal instead. For example, tion decomposition of solids has been pointed the average oxidation number of the oxidizing out a1r~ady.I~The presence of these procrsscs bromine species in the crystals was much lower in the alkali metal bromates must be rccognizcd. (e.g., 2.0 to 3.0 meq./mmole) than that expected Acknowledgments.-It is a pleasure to acknowl(e.g., 6.0 meq./mmole) if bromate free radicals or edge the assistance given by several members if Br02 were present (Table 111). It was assumed of the ORNL Analytical Chemistry Division in in (2A-C) that negatively charged bromine species various phases of our researches: D. E. LaValle and ncutral oxygen atoms were formed. The for the synthesis of the 'LiBr03, llbBrOs arid electron a f % n i t i e for ~ ~ ~atomic bromine (3.54 e.v.) CsBrOa preparations and C. A. Pritchard for the> and oxygen (2.31 e.v.) are such as to favor the flame photometric analysis of the lattcr two comformation of Br- and 0 rather than Br 0-. pounds; C. M. Boyd for the DTA and T G R Values for the other bromine oxidation states are examinations and R. Sherman for the X-ray unknown, but, if their electron affinities are nearly powder pattern identifications of all the salts; the same as for C102 and C10, respectively, then A. S. Myers and I. Rubin for t h r micro-oxygen BrOz- and BrO- will be favored over BrOz and analyses on the irradinlcd crystals; T. E. Willmarth nro. for the optical and electron microscope stiidicis ( 2 ) The abstraction reaction 3 would be ex- and C. A. I-Iorton for thc prcliminnry infrarcd d to be temperature and dose rate dependent. measurements. Br02- +Br-
+ 02
(5)
+
(21) 11. 0 I'iitclraid. C h m . Reus., 63, 629 (1933).
( 2 5 ) A.
R. Jones, Sczence, 137,234 (1958).
